The present invention generally relates to pulleys and pulley structures, and more particularly to a pulley and a pulley structure capable of preventing deformation of an external form of the pulley.
For example, a press-fit pulley structure is used as a capstan of a magnetic tape recording and/or reproducing apparatus and a driving force transmitting member of a magnetic disc recording and/or reproducing apparatus. For convenience' sake, a description will hereunder be given for the latter case.
FIG. 1 generally shows a magnetic disc recording and/or reproducing apparatus (hereinafter simply referred to as an apparatus) employing the conventional pulley structure. In FIG. 1, the apparatus comprises a head seeking motor 11, a head carriage 12, a magnetic head 13 for recording and/or reproducing a signal on and/or from a magnetic disc 14, a pulley 15, a motor shaft 16, and a belt 17. The belt 17 is wrapped around the pulley 15 in the form of the letter alpha, and both ends of the belt 17 are fixed to the head carriage 12.
When the motor 11 is driven and the pulley 15 rotates, the belt 17 is taken up by the pulley 15 to move the head carriage 12 in a direction of an arrow A. As a result, the magnetic head 13 is positioned on a predetermined track T of the magnetic disc 14.
As may be seen from FIG. 1, a deformation of the external form of the pulley 15 directly affects the positioning accuracy of the magnetic head 13, and this positioning accuracy becomes poor when the deformation occurs. This is a serious problem for the apparatus which must carry out a high density recording.
In addition, in the case of the capstan, the deformation of the external form of the capstan directly affects the transport characteristics of the magnetic tape, and an accurate signal recording and/or reproduction cannot be carried out when the deformation occurs.
The pulley 15 is generally fixed on the motor shaft 16 by a press-fitting as shown in FIG. 2. In FIG. 2, M denotes a difference between a diameter of the motor shaft 16 and an internal diameter of a center hole of the pulley 15, that is, an extra thickness, for ensuring a tight fit of the pulley 15 to the motor shaft 16. The pulley 15 employing this press-fit structure must be such that the deformation of the external form of the pulley 15 does not occur easily.
An example of the conventional pulley structure employing the press-fitting is shown in FIGS. 3A and 3B. As shown in FIG. 3A, a pulley 15A is press-fit on the motor shaft 16 for the entire length of the pulley 15A. According to this structure, the extra thickness of the pulley 15A covers the entire length of the pulley 15A, and the external form of the pulley 15A becomes considerably deformed when the pulley 15A is press-fit on the motor shaft 16, as indicated by a solid line in FIG. 3B. In FIG. 3B, a one-dot chain line indicates the external form of the pulley 15A before being press-fit on the motor shaft 16. For this reason, the head positioning accuracy of the apparatus shown in FIG. 1 becomes poor when the pulley 15A is employed.
In addition, the motor shaft 16 is generally made of stainless steel, for example, and the pulley 15A is made of aluminum alloy, for example. The coefficient of thermal expansion of the stainless steel is smaller than that of the aluminum alloy, and the coefficient of thermal expansion differs between the motor shaft 16 and the pulley 15A. Hence, in the case where the apparatus is used at an ambient temperature lower than room temperature, the pulley 15A contracts more than the motor shaft 16. However, since the entire inner peripheral surface of the pulley 15A at the center hole thereof is in close adherence with the outer peripheral surface of the motor shaft 16, the deformation of the pulley 15A caused by the temperature change to the low ambient temperature cannot be absorbed at the inner portion of the pulley 15A and appears at the external portion of the pulley 15A. As a result, the external form of the pulley 15A becomes deformed as indicated by a phantom line in FIG. 3B, and the head positioning accuracy of the apparatus becomes even further deteriorated.
FIG. 4 shows another example of the conventional pulley structure in which a pulley 15B is press-fit only partially on the motor shaft 16. In FIG. 4, L1 indicates a press-fit portion and L2 indicates a non-fit portion. The external form of the pulley 15B at the non-fit portion L2 is uneasily deformed, and for this reason, the belt 17 is wrapped on this non-fit portion L2.
According to the pulley 15B, it may not be possible to set a length a of the pulley 15B to a desired length due to space limitations within the apparatus. In this case, the belt 17 is inevitably wrapped around the pulley 15B at a portion exceeding the non-fit portion L2 and reaching the press-fit portion L1. Because the deformation occurs at the press-fit portion L1 at a low temperature similarly as in the case described before, the problems of the pulley 15A shown in FIGS. 3A and 3B are also introduced with the pulley 15B. Furthermore, since the length of the press-fit portion L1 is short, the pulley 15B easily vibrates at the non-fit portion L2.
FIG. 5 shows a conceivable pulley structure in which a narrow portion 16a is provided at an intermediate part of the motor shaft 16 and a pulley 15C is press-fit on the motor shaft 16 by avoiding contact with the motor shaft 16 at a central portion thereof. In other words, only the end portions of the pulley 15C are secured on the motor shaft 16 by the press-fitting. This pulley 15C will not easily vibrate at the end portion as in the case of the pulley 15B. However, the external form of the pulley 15C at press-fit portions 15Ca and 15Cb thereof becomes deformed at the low temperature, and in this case, the external form of the pulley 15C becomes irregular along a longitudinal direction thereof. Therefore, problems similar to those already described also occur according to the pulley 15C.